ON THE COLOUES OF THICK PLATES. 157
Newton, between two object-glasses laid upon one another," Sir William Herschel mentions an experiment in which rings of the nature of those of thick plates were produced by merely strewina-hair-powder in the air in front of a metallic speculum placed as the mirror in Newton's experiment*. The result of this experiment was justly regarded by Herschel as inexplicable on the theory of fits. It may here be remarked that it is in perfect accordance with the theory of undulations.
In the Annales de Ghimie et de Physique-^, will be found a report by Ampere and Poisson on a memoir by M. Pouillet, containing some experiments on the rings. The experiments were mostly the same as those of the Duke de Chaulnes, but accompanied by measures. M. Pouillet found that the rings were produced by placing in front of a metallic speculum an opaque screen containing an aperture of any form. In this case the rings were round, whatever might be the form of the aperture. The experiments are mentioned by M. Pouillet in his Siemens de Physique^.
A complete explanation of the rings, according to the theory of undulations, has been given by Sir John Herschel in his treatise on Light§. The rings are supposed to be formed in Newton's manner with a glass mirror, the luminous point being situated in the axis. Having investigated the elementary system of rings which would be produced by the two streams scattered in passing and repassing at the point of the first surface where it is cut by the axis, Sir John Herschel shews that if the surfaces be supposed to be a pair of concentric spheres, having the luminous point in their centre, the elementary systems corresponding to the several elements of the first surface will be superposed, in such a manner that a distinct system would be thrown on a screen held at the distance of the luminous point. The laws of the rings resulting from theory are precisely those which had been discovered by Newton, and the calculated magnitudes were found to agree almost exactly with Newton's measures.
A set of coloured hands have since been observed by Dr Whcwoll, which arc formed when the image of a candle held near
* Philosophical Transactions for 1807, p. 231.
t Tom. i. (1816) p. 87. -1: Tom. n. p. 476.
§ EncyclojMdia Metropolitana., Arts. 076, &c.